This invention relates to voltage and current sensing.
Capacitively-coupled voltage measurement is frequently used to measure the voltage present on a high voltage conductor in high voltage alternating current electric systems. Typically, a high voltage capacitor is connected between the high voltage conductor and the secondary winding, and a load capacitor is connected between the secondary winding and the toroidal ferro-magnetic core. The high voltage capacitor and the load capacitor form a simple capacitive voltage divider from which the voltage of the high voltage conductor may be determined. Voltage measurement is often supplemented with a measurement of current flowing through the high voltage conductor. Typically, a current transformer is used to provide this current measurement by surrounding the high voltage conductor with a ferro-magnetic transformer core around which an insulated secondary winding is wound uniformly.
Although capacitively-coupled voltage sensing is widely used, the cost and precision of it the capacitively-coupled sensors are closely related to the quality of the high voltage capacitors used to perform the measurements. High precision is often achieved by using closely matched foil capacitors immersed in a dielectric liquid or ceramic capacitors built with high-performance, temperature-compensating materials. These high precision capacitors generally are quite expensive.
A low cost approach is achieved by constructing a voltage-sensing capacitor as an integral part of the high voltage apparatus. The capacitance of such a capacitor is determined by the internal device geometry and the dielectric constant of an associated insulating material. The low cost approach often produces a relatively low capacitance value that limits the overall measurement accuracy of the design. Low capacitance, and therefore low energy, also presents a challenge in transmitting the measured information from the sensor to the device that is performing the voltage measurement.
Parasitic capacitance between the current transformer secondary winding and the high voltage conductor may elevate the potential of the secondary winding, which may lead to failure of the secondary winding insulation. A similar problem applies to the ferro-magnetic based transformer core if the potential is left freely floating with respect to the high voltage conductor potential. To reduce or eliminate this current transformer failure mechanism, the standard approach has been to ground the current transformer core or to add a grounded shielding electrode that protects the current transformer secondary winding.
In one general aspect, improved precision for the measurement of the AC voltage applied to a primary high voltage conductor of a desired phase in a multi-phase system is achieved by using active electronic circuitry to compensate for crosstalk introduced by one or more additional phases in the multi-phase system. The voltage measurement may be based upon the use of a capacitive voltage sensor for each phase in the multi-phase system. A voltage measurement is obtained for the desired phase and for each additional phase in the multi-phase system. A product is generated for each additional phase by multiplying the additional phase voltage measurement by a corresponding predetermined constant. The product for each additional phase then is subtracted from the voltage measurement of the desired phase.
Implementations may include one or more of the following features. For example, in a three-phase system, there is a desired phase and two additional phases. A first product is generated for a first additional phase and a second product is generated for a second additional phase, and the products then are subtracted from the desired phase.
In another general aspect, improved precision for the measurement of the AC voltage applied to primary high voltage conductors of a multi-phase system such as a three-phase system is achieved by using active electronic circuitry to compensate for factors such as capacitive sensor gain, output impedance, and crosstalk limitations where the voltage measurements are based upon the use of a capacitive voltage sensor for each phase in the multi-phase system. A capacitive voltage divider having a first and second capacitance may be associated with each individual phase, with a drain resistor connected in parallel with the second capacitance for the associated phase. A high impedance amplifier, a programmable gain stage, a memory storage, a temperature compensating circuit, and a crosstalk cancellation circuit are used in connection with the capacitive voltage divider for each phase.
Implementations may include one or more of the following features. For example, in a three-phase system, outputs of three voltage sensors are amplified, corrections are made for gain and temperature variations, and the outputs are combined together to cancel the effects of mutual coupling between the three individual phases. A signal processing circuit is capable of amplifying and buffering the individual phase voltage sensor signals, adjusting the sensor output level, and producing an output capable of sending the measured signals to a remote control unit. In another example, a differential output driver and surge protection network are connected to assist in generating a balanced, surge-protected, low-impedance output which may be sent to a remote control unit. Also, a calibration port may be connected to the memory storage. In another example, a surge suppressor may be connected in parallel with the second capacitance of the capacitive voltage divider.
In another example, the crosstalk cancellation circuit may have an operational amplifier, a connecting resistor connected between the input and the output of the operational amplifier, a voltage input for the desired phase and each additional phase of the multi-phase system, and a resistor associated with each additional phase of the multi-phase system. In a three-phase system, for example, there would be a desired phase and two additional phases.
Other features and advantages will be apparent from the description and drawings, and from the claims.